A circuit for generating pulse width modulated (PWM) pulses accurately by having ramp and discharge times that are constant regardless of the PWM timing of the output, and a feedback loop to maintain the duty cycle over a range of clock frequencies.
A PWM circuit normally compares a ramp waveshape having a constant slope against a variable threshold DC level to produce an output pulse that toggles at the point where the ramp level exceeds the threshold level. Also, as soon as the threshold level is reached, the ramp is discharged to zero. For long output pulses in relatively high speed circuits, the time for the ramp circuit to discharge is short, and any variability in the ramp slope or threshold voltage results in a variation in this discharge time, which in turn, affects the timing of the next cycle. Thus, the variability of the discharge time of the circuit results in significant PWM tolerances that make it unusable in high speed precision applications such as the modulation of video data.
Also, the ramp waveshape actually has to go a bit past the threshold level for the comparator to be able to react to it. This additional amount of drive is called "overdrive", and the comparator output delay is highly influenced by the amount of this overdrive at its input. Overdrive less than a certain minimum value results in more delay between the time of the crossover point and the fall time of the output pulse. In current designs this amount is not controlled, thereby resulting in inconsistent widths for the same threshold levels. However, due to the over-all period mandated by some design requirements (such as for switching power supply applications) the variabilities of the device delays are considered insignificant. However, an uncontrolled amount of overdrive can result in timing tolerances that are unacceptable, especially in applications where these uncertainties are significant as compared to the over-all period of the data being modulated.
A third problem that a PWM circuit must deal with is that of a changing clock frequency. If the output pulse ON time is fixed and the clock frequency changes, the OFF time, and therefore the duty cycle, also changes. Fourth, if the ramp is discharged at the crossover point, the circuit can only be used to generate one output pulse train. Finally, the circuit must be as simple, and use as few parts, as possible.